How do the forces of air and water resistance and lift affect the movement of performers and projectiles?
The factors affecting air resistance and drag, the Bernoulli principle and the production of lift, the Magnus effect on spinning balls, and the techniques used to minimise drag and maximise lift in sport.
A focused answer to AQA A-Level PE biomechanics on fluid mechanics, covering the factors affecting air resistance and drag, the Bernoulli principle and lift, the Magnus effect on spinning balls and techniques to minimise drag and maximise lift.
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What this dot point is asking
AQA wants you to explain the factors affecting air resistance and drag, apply the Bernoulli principle to the production of lift, explain the Magnus effect on a spinning ball, and describe how performers minimise drag and maximise lift to improve performance.
Air resistance and drag
It helps to distinguish the two main forms of drag AQA expects. Surface drag (friction or skin drag) arises from friction between the fluid and the surface of the body, and is reduced by smooth, tight clothing and a smooth surface. Form drag (pressure drag) arises from the pressure difference between the front and the turbulent low-pressure wake behind a body, and is reduced by streamlining the shape so the airflow stays attached and the wake is smaller. A racing cyclist crouching low tackles both: a smaller frontal area cuts form drag and a smooth suit cuts surface drag. Counter-intuitively, the dimples on a golf ball deliberately create a thin turbulent boundary layer that clings to the surface, delaying separation and reducing the size of the wake, so a dimpled ball actually has less form drag and flies further than a smooth one.
The Bernoulli principle and lift
The Bernoulli principle states that where a fluid flows faster, it exerts lower pressure, and where it flows slower it exerts higher pressure. When air travels faster over one surface of a shape than the other, the pressure difference produces a force, called lift, towards the low-pressure side.
A discus angled correctly creates faster airflow over the top and slower flow underneath, so the higher pressure below produces lift that keeps it in the air longer. The same principle gives an aerofoil its upward lift. In some events, such as a downhill skier, a downward lift force can be used to improve grip and stability.
The Magnus effect
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20214 marksExplain, using the Bernoulli principle, how a ski jumper uses body position to maximise the distance jumped.Show worked answer →
AO1/AO2. The ski jumper angles the body and skis to act like an aerofoil. Air travelling over the upper surface moves faster and over the lower surface slower. By the Bernoulli principle, the faster flow over the top exerts lower pressure and the slower flow underneath exerts higher pressure. This pressure difference produces an upward lift force that opposes weight, keeping the jumper in the air longer and increasing horizontal distance. A strong answer notes that the jumper adopts the V-shape to increase surface area and a low angle of attack to balance lift against drag. Reward the faster-flow-lower-pressure statement, the named lift force, and the link to a longer flight.
AQA 20183 marksDescribe the Magnus effect and explain why a football struck with sidespin curves in flight.Show worked answer →
The Magnus effect is the deviation in the flight path of a spinning ball caused by a pressure difference set up by the spin. With sidespin, one side of the ball spins in the same direction as the airflow (air speeds up, pressure falls) and the other side spins against it (air slows, pressure rises). By the Bernoulli principle the resulting pressure difference pushes the ball from the high-pressure side towards the low-pressure side, so it curves sideways in flight (the bend on a free kick). Reward identifying the faster airflow and lower pressure on the side the ball curves towards, and linking spin direction to curve direction.
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Sources & how we know this
- AQA A-level Physical Education (7582) specification — AQA (2016)